feat: Add model to store as source of truth

[lornakelly]

Description

Dependency: #48

Add the parsed workflow model to the store as the single source of truth for the diagram editor. The store should expose a way to set and clear the model, and be accessible to components throughout the editor. For the read-only MVP, the store needs to handle updating the model when the content prop changes and clearing it when the component unmounts.

Motivation

The diagram editor needs a central place to hold the workflow model so that all components can access it without prop drilling.

Proposed Implementation

No response

Definition of Done

• Implementation: Fully implemented according to the Serverless Workflow spec.
• Unit Tests: Comprehensive unit tests are included and passing.
• Integration Tests: Verified within the monorepo and target environments (Web/VS Code).
• Documentation: Updated README.md, ADRs, or official docs.
• Performance: No significant regression in editor responsiveness.
• Accessibility: UI changes comply with accessibility standards.